1 research outputs found
The lives and deaths of positrons in the interstellar medium
We reexamine in detail the various processes undergone by positrons in the
ISM from their birth to their annihilation using the most recent results of
positron interaction cross sections with H, H2 and He. The positrons' lives are
divided into two phases: the 'in-flight' phase and the thermal phase. The first
phase is treated with a Monte Carlo simulation that allows us to determine the
fraction of positrons that form positronium and annihilate as well as the
characteristics of the annihilation emission as a function of the medium
conditions. The second phase is treated with a binary reaction rate approach,
with cross sections adopted from experimental measurement or theoretical
calculations. An extensive search and update of the knowledge of positron
processes was thus undertaken. New reaction rates and line widths have been
obtained. We investigate the treatment of the complicated interactions between
positrons and interstellar dust grains. New reaction rates and widths of the
line resulting from the annihilation inside and outside of the grain have been
obtained. The final results of our calculations showed that dust is only
important in the hot phase of the ISM, where it dominates all other processes.
Combining the new calculations, we have constructed annihilation spectra for
each phase of the ISM, considering various grain contents, as well as an
overall combined spectrum for the ISM as a whole.Comment: 16 pages, 6 figures. accepted in Astronomy and Astrophysic